RU2007851C1 - Class d amplifier on tetrode - Google Patents

Abstract

FIELD: radio engineering. SUBSTANCE: this amplifier is meant for radio transmitters. It includes signal former with pulse-duration modulation, powerful amplifier based on tetrode and load. Output of cathode follower supplied from source of screen voltage is linked to screen grid of tetrode. Thanks to it voltage across screen grid changes in accordance with amplified signal. This leads to reduced current of screen grid and to diminished losses across it. EFFECT: reduced power dissipated in circuit of screen grid of tetrode. 2 dwg

Description

 The invention relates to radio engineering and can be used in high-power radio transmitters as a modulator.

 The use of key amplifiers with pulse width modulation (PWM) as modulators allows to increase the efficiency of transmitters, to abandon the use of bulky modulation transformers, and also to amplify the envelope of amplitudes of a single-band signal in high-performance radio transmitters with single-band modulation. At the same time, tetrodes are used as key elements in most practical schemes of powerful PWM modulators.

 Of the known key modulators, the closest in their technical essence is a single-cycle PWM amplifier, where a tetrode is used as a key element, on the control grid of which a PWM signal is supplied, i.e., rectangular pulses whose duration varies in accordance with the instantaneous value of the modulating voltage. The amplified PWM voltage from the tetrode anode through a low-frequency filter, which restores the modulating signal, enters the load, the role of which is played by the terminal stage of the radio transmitter.

 Tetrodes require less power in the circuit of the control grid to excite than triodes, but when using tetrodes, it becomes necessary to choose the lamp operating mode so that the loss power on the screen grid does not exceed the permissible level. This is especially significant when using the tetrode in the key mode, when a significant part of the period of the switching frequency of the tetrode is in saturation. In this case, the value of the residual voltage at the anode is small, therefore, the current of the screening grid and the amount of power dissipated on it sharply increase.

 Significant power losses on the tetrode shielding grid are a disadvantage of the prototype, which reduces the lamp life and also limits the output power of the PWM amplifier. Known measures to limit the current of the screening grid using a choke practically do not reduce the amount of power dissipated on the screening grid of the tetrode of a single-cycle PWM modulator of the radio transmitter in silent mode, when the duty cycle of the pulses is 2 and the voltage at the output of the modulator is half the maximum value, while Calculations using the nonlinear mathematical model of the tetrode show that it is in this mode that the average power loss in the circuit of the screening grid is close to the maximum.

 The purpose of the invention is to reduce the power dissipated in the circuit of the tetrode screening grid.

 This is achieved by the fact that in an amplifier with a PWM on a tetrode containing a signal generator with a PWM, the output of which is connected to the control grid of the tetrode, a screen voltage source whose negative bus is grounded, the cathode of the tetrode is also grounded, the anode of the tetrode is connected to the anode of the diode, the cathode of which is connected to the positive bus of the anode power source, the negative bus of this source is grounded, the first load taps of the amplifier and the low-pass filter capacitor are connected to the positive bus of the anode power source, their the second taps are connected together and connected to the tetrode anode through the filter inductor, the positive bus of the screen voltage source is connected to the cathode follower power supply bus, the output of which is connected to the tetrode shielding network, the input of the signal generator with PWM, which is the input for the amplified signal, is connected to the first input the adder, to the second input of which a constant bias source is connected, and the output of the adder through a DC amplifier is connected to the input of the cathode follower.

E

=

, (1) где U_{вх макс} - максимальное значение усиливаемого однополярного сигнала U_вх.The introduction of new features entails new properties of the amplifier, which consist in the fact that the voltage on the screening grid of the tetrode l _c2 smoothly varies according to a linear law in accordance with the value of the analog signal at the input of the amplifier U _in . In this case, the maximum value of the voltage on the shielding grid E _{s2max is} approximately equal to the voltage at the output of the screen voltage source, and the initial bias of the adjustment characteristic E ₁ is set by appropriate selection of the gain factor K of the DC amplifier having zero initial current and the output voltage of the bias source E ₁ ^l
K =

E

=

, (1) where U _{in max} is the maximum value of the amplified unipolar signal U _in .

The maximum value of the voltage on the screening grid E _c2 max is selected based on the possibility of ensuring a sufficiently high modulator efficiency at the peak modulation point (with U _in = U _{in max} ). An increase in the voltage on the shielding grid of the lamp of the key amplifier leads to a decrease in the residual voltage at the anode, therefore, an increase in the efficiency of the anode circuit of the modulator (the condition E _{c2 max} <E _{c2 dop} , where E _{c2 dop} is the permissible voltage value on the shielding grid must also be observed).

Calculations show that when using modern powerful tetrodes, the initial bias value E ₁ can be chosen close to zero, which has almost no effect on the shape of the static characteristic of a PWM amplifier, which is the dependence of the DC voltage at the load on the value of the DC input voltage, therefore, the level non-linear distortion of the output signal also does not change. In this case, the average current and losses in the circuit of the shielding grid as compared with the case of unregulated screen voltage l _c2 = E _{s2 max} (i.e., compared to the prototype) in the silent mode are reduced by about half, and only half of the indicated power is dissipated on the shielding grid of the tetrode (the rest is dissipated in the cathode repeater), i.e., the power dissipated on the shielding grid is reduced by about 4 times compared with the prototype.

 In FIG. 1 shows the dependence of the voltage on the screening grid of the tetrode on the voltage at the input of the amplifier; in FIG. 2 is a functional diagram of the proposed PWM amplifier.

 The functional circuit includes a PWM signal generator 1, tetrode 2, screen voltage source 3, diode 4, amplifier load 5, low-pass filter capacitor 6, anode power supply 7, filter inductance coil 8, cathode follower 9, adder 10, voltage source 11 bias and DC amplifier 12.

 The output of the former 1 is connected to the control grid of the tetrode 2, the negative bus of the source 3 is grounded, the cathode of the tetrode 2 is also grounded, the anode of the tetrode is connected to the anode of the diode 4, the cathode of which is connected to the positive bus of the source 7, the negative bus of this source is grounded, the first load taps the amplifier 5 and the capacitor 6 of the low-pass filter are connected to the positive bus of the source 7, and their second taps are connected together and through the coil 8 of the filter inductance are connected to the anode of the tetrode 2, the positive bus of the source 3 p connected to the power bus of the cathode repeater 9, the output of which is connected to the shielding grid of the tetrode 2, the input of the driver 1, which is the input for the amplified signal, is connected to the first input of the adder 10, to the second input of which the bias voltage source 11 is connected, and the output of the adder 10 through the amplifier 12 is connected to the input of the cathode follower 9.

 The device operates as follows.

The modulating signal U _in goes to the input of the PWM signal generator 1, which generates voltage pulses on the control grid of the tetrode 2, the duration of which is proportional to the value of the modulating signal. Accordingly, the voltage at the anode of tetrode 2 also has the shape of a PWM pulse. The component of this voltage that changes in accordance with the modulating signal is extracted by a low-pass filter, consisting of an inductor 8 and a capacitor 6, and enters the load of the amplifier 5. The end stage of the radio transmitter in which the anode modulation is performed plays a role of the load. The anode circuit of tetrode 2 is powered from a voltage source 7. Diode 4 closes the current of the inductor 8 at times when tetrode 2 is locked.

The input signal of the modulator U _Ix is also fed to one of the inputs of the adder 10, where it is summed with the output voltage of the bias source 11, and the total signal is amplified to the required value by the DC amplifier 12 (the values of the gain K of the amplifier 12 and the voltage of the bias source E ₁ ^{l are} determined by the formulas ) and through the cathode follower 9, powered by a screen voltage source 3, is supplied to the screening grid of tetrode 2.

 The maximum voltage on the shielding grid of high-power tetrodes is in the range of 1-2 kV, and the power dissipated on the shielding grid is on the order of units of kW, therefore a relatively low-power lamp can be used in the cathode repeater.

 The technical and economic efficiency of the invention is determined by the fact that, thanks to it, it is possible to reduce the power dissipated on the shielding grid of the tetrode used as a key element in comparison with the prototype several times, therefore, it can either increase the life of the tetrode or significantly increase the output power of the radio transmitters from Highly efficient PWM modulators. (56) Foreign radio transmitting devices. / Ed. G.A. Zeitlenka and A.E. Ryzhkov. M.: Radio and Communications, 1989, p. 26, fig. 2.12.

Claims (1)

 CLASS D AMPLIFIER ON A TETROD, comprising a pulse-width modulated signal former, the output of which is connected to a tetrode control grid, the cathode of which is connected to the ground bus, and the anode to the anode of the recovery diode and through the low-pass filter choke to the first terminals of the load and capacitor low-pass filter, the second terminals of which are connected to the cathode of the recovery diode and the anode supply bus, as well as a screen voltage source, the negative bus of which is connected to the earth bus, characterized in then, in order to reduce the power dissipated in the tetrode screen grid circuit, a cathode follower is introduced, the power output of which is connected to the positive bus of the screen voltage source, and the output is connected to the tetrode screen grid, the adder and the bias voltage source, the output of which is connected to the first input of the adder the second input of which is connected to the input of a pulse-width modulated signal conditioner, and the output of the adder is connected to the input of the cathode follower through an input DC amplifier.

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